This invention relates to charged particle beam lithography and, more particularly, to an exposure system in which scanning is performed by a high resolution variable length line.
Electron beam exposure systems are used commercially for selectively irradiating a resist-coated workpiece to define the features of a semiconductor device. The workpiece can be a mask plate or can be a resist-coated semiconductor wafer in which features are defined directly. In either case, an electron beam is controlled in a highly accurate, high speed manner to expose microminiature patterns in the electron resist material.
Various approaches have been taken in controlling the electron beam. A small circular spot beam can be raster scanned over the entire surface of the workpiece and turned on or off to produce the desired pattern. A system using this approach is disclosed in U.S. Pat. No. 3,900,737, issued Aug. 19, 1975, to Collier et al. Alternatively, the spot beam can be directed to desired pattern areas and scanned only over those pattern areas in a vector scanning approach. Either approach is relatively slow since the area covered by the spot beam at any instant is extremely small. In another system, the electron beam is shaped into rectangles of variable shape and size. The rectangles are used to flash expose successive areas of the pattern. Such a system is described by H. Pfieffer in "Variable Spot Shaping for Electron-Beam Lithography," J. Vac. Sci. Technol., Vol. 15, No. 3, May/June 1978, p. 887. One drawback of variable shaped rectangle systems is the difficulty in exposing angled or odd-shaped pattern features.
In yet another approach, an elongated rectangular beam is scanned in a direction perpendicular to its long dimension. As the beam is scanned, the length of the rectangle is varied to define a desired pattern. This approach permits patterns of almost any shape to be exposed in a single operation. In producing a variable shaped beam, an image of a first square aperture is focused on a second square aperture. Shaping deflectors dynamically position the image of the first aperture relative to the second aperture so as to provide a beam cross-section of the desired length and width.
Electron beam exposure systems typically have the capability of exposing patterns with features of one micrometer or less. In such systems, even small inaccuracies in the size or position of the electron beam can seriously degrade system performance. One known source of electron beam inaccuracy is the coulomb repulsion between electrons in the beam. Repulsion between electrons along the axis of the beam causes a spread in electron energies and is known as the Boersch effect. See, for example, W. Knauer, "Boersch Effect in Electron-Optical Instruments," J. Vac. Sci. Technol., Vol. 16, No. 6, Nov./Dec. 1979, p. 1676. Repulsion between electrons transverse to the axis of the beam causes radial spreading of the beam. See, for example, E. Goto et al., "Design of Variable Shaped Beam Systems," Proc. of the 8th Int. Conf. on Electron and Ion Beam Science and Technology, 1978, p. 135, and T. Groves et al., "Electron-Beam Broadening Effects Caused by Discreteness of Space Charge," J. Vac. Sci. Technol., Vol. 16, No. 6, Nov./Dec. 1979, p. 1680. Either type of repulsion causes loss of resolution in the system. The radial spreading contributes directly to loss of resolution. Energy spreading results in loss of resolution since electrons with different energies undergo different deflections as they pass through lenses and deflectors.
It has been determined that energy spreading occurs principally at the electron beam crossover points. Radial spreading, however, occurs progressively along the length of the electron beam and is approximately proportional to the beam current and to the length over which the coulomb repulsion occurs. Therefore, in order to minimize radial beam spreading, it is desirable to minimize both the length of the electron optical column and the beam current.
Conflicting with these requirements is the necessity for irradiating the workpiece with the highest possible electron beam current in order to achieve high speed operation.
As noted hereinabove, prior art systems have utilized two square apertures for shaping the beam into a rectangle of variable length. The current between the two apertures is limited only by the first square aperture. Other prior art systems have employed three square shaping apertures to trim the beam to the desired size and shape and reduce the effect of radial beam spreading. A disadvantage of this approach is the added complexity in the electron optical column. In addition to the third aperture, an additional lens and deflector are required. A further disadvantage is the increase in column length necessitated by the additional elements.
Apertures of various shapes have been used in electron optical columns. Character-shaped apertures are used to project characters directly on a workpiece in U.S. Pat. No. 4,213,053, issued July 15, 1980 to Pfieffer. Apertures for a four-spot raster scanning system are shown by M. Thompson et al., in "Double-Aperture Method of Producing Variably Shaped Writing Spots for Electron Lithography," J. Vac. Sci. Technol., Vol. 15, No. 3, May/June 1978, p. 891. However, neither of these references is related to scanning with a line of variable length.
It is a general object of the present invention to provide a new and improved charged particle beam exposure system for selective irradiation of a workpiece.
It is another object of the present invention to provide a charged particle beam exposure system wherein pattern exposure is performed by scanning with a high resolution line of variable length and controlled width.
It is yet another object of the present invention to provide a charged particle beam exposure system wherein beam spreading is reduced.